1. Field of the Invention
This invention relates to a direct-coupled DC amplification circuit. More particularly, the present invention is directed to provide an amplification circuit which effects amplification over all the amplification stages using a ground potential as the reference voltage, and which affords an extremely high power source noise suppression ratio up to a high frequency range.
2. Description of the Prior Art
An amplification element forming a second or subsequent amplification stage of a heretofore known direct-coupled amplification circuit generally makes use of a power source voltage as its reference voltage. Accordingly, the amplification element amplifies the difference between a signal voltage delivered from the preceding initial or second stage and the power source voltage. When the amplification element is composed of transistors (FETs or other transistors), even if the power source voltage is superposed with low frequency components, the influence of the low frequency components is negligible with respect to the output, because the output impedance of the element is low. Hence, a sufficient power source noise suppression ratio can be obtained at least in a low frequency range. The fact that negative feedback (NF) quantity of the amplifier as a whole is generally greater at low frequencies, also contributes to the improvement in the power source noise suppression ratio in the low frequency range.
In an amplification stage having the highest voltage gain and the lowest pole in the amplification circuit, design is made so that the pole is further reduced by a phase compensating capacitor to maintain stability of the circuit as a whole.
The operation of a conventional second or subsequent amplification stage including the phase compensating capacitor will here be described. As shown in FIG. 1, this amplification stage includes two transistors Q.sub.1, Q.sub.2 that are cascode-connected to each other, and the phase compensating capacitor C is interposed between the collector of the transistor Q.sub.2 and the base of the transistor Q.sub.1. The emitter of the transistor Q.sub.1 is connected to the positive power terminal. A buffer transistor for increasing an output current or the like is connected as a load R.sub.L.
In the amplification stage having the above-mentioned construction, when the frequency exceeds its pole (e.g., from several hundred Hertz to several tens of thousands Hertz) and the capacitor effect comes to appear, the transistors Q.sub.1, Q.sub.2 function as a kind of inverting feedback circuit, thereby forming the pole. Since negative feedback is applied from the collector of the transistor Q.sub.2 to the base of the transistor Q.sub.1 via the capacitor C, the capcitor describes a -6 dB/OCT curve. This is the intended purpose of this capacitor C. The problem here is that since the emitter of the transistor Q.sub.1 is connected to the power terminal, the power source voltage serves as the reference potential for the amplification stage consisting of the transistors Q.sub.1 and Q.sub.2. If the capacitor C is not included, there is no relation between the base voltage of the transistor Q.sub.1 and the output impedance of the transistor Q.sub.2, because the latter is extremely high. When, however, the capacitor C is disposed as illustrated in the drawing, the potentials at the emitter of the transistor Q.sub.1, at the collector of the transistor Q.sub.2 and at the base of the transistor Q.sub.1 are closely associated with one another in the high frequency range in which negative feedback is applied through the capacitor C.
In the frequency range sufficiently higher than the pole of the amplification stage and including a flat practical band in the output characteristics of the amplifier, the output voltage .DELTA.V of the amplification stage is given by the following euqation: ##EQU1## where .DELTA.i is a current fed, C is capacitance of the capacitor, .omega. is angular frequency and .DELTA.v represents noise components contained in the power source voltage.
As is obvious from the above equation, the signal voltage ##EQU2## in the output voltage decreases with an increasing frequency ##EQU3## In contrast, though the noise component (.DELTA.v) is never amplified so as to exceed 1 by the transistors Q.sub.1, Q.sub.2, it is independent of the frequency and so is always contained in the output voltage .DELTA.V. In order to suppress the influences of the noise component .DELTA.v on the output voltage, therefore, it is necessary to apply the negative feedback to the amplifier as a whole, so that the noise suppression ratio may be represented by a reciprocal of the total negative feedback quantity. The noise suppression ratio decreases as the frequency becomes higher, because the negative feedback quantity in the amplifier as a whole drops in the frequency range higher than the pole formed by the amplification stage.
It has conventionally been believed generally, that an increased operation speed and a broader band of the amplification circuit are necessary factors for obtaining a high power source noise suppression ratio up to the high frequency range. It is certainly true that in the conventional circuit, the signal component (.DELTA.i/.omega.C) must be increased with respect to the noise component .DELTA.v in order to enhance the power source noise suppression ratio. More definitely, this can be accomplished by increasing the current fed .DELTA.i and reducing the capacitance C. In addition, large quantities of negative feedback may be applied up to the high frequency range. These directly result in the increase in the operation speed of the amplifier and broadening of its band. As for the amplifiers, however, there are a wide range of applications in which the broadening of the band and the increase of the operating speed are not really necessary, and only the power source noise suppression ratio needs to be increased. In other words, an amplifier having an unnecessarily broad band and an unnecessarily high operation speed must often be employed merely in order to obtain a high power source noise suppression ratio. However, such an amplifier is generally not so easy to use because it has the problems that oscillation is likely to occur and unnecessary waves are picked up due to its broader band.
When the output band is relatively narrow as in active band-pass filters or the like, the power source noise may seem reduced even when an ordinary amplifier is used, because the pass band is narrow. In practice, however, the noise mixes inside the circuit and is as such delivered as the output. In other words, a broader band amplifier becomes necessary in this case, too, in order to reduce the influences of the power source noise.
As can be understood clearly from the foregoing description, the power source noise suppression ratio in the conventional amplification circuit is closely related with the dynamic characteristics (slew rate or the like) of the circuit or with its open loop characteristics and hence, it has generally been believed that the noise suppression ratio can not be discussed separately from these characteristics.